FORESTS AND THE WATER SUPPLY.
[Specially written for FIRS AND WATER.]
JUST at present when the question as to the future of water supply of the Greater New York is being raised and that of preventing the denuding of the Adirondack region—one of the proposed sources— of its trees is being mooted, it will not be out of place to consider the relation existing between forests and the water supply. That the clearance of the land from forests has a perceptible influence upon the climate goes without saying; that it -ffectH the water flow is equally in evidence. The question is what influence has this denudation of forest land upon the water supply in this and other States,and especially in those dependent upon irrigation for their prosperity. The solution must depend in great measure upon local conditions—the character, extent, and specific nature of the forest cover. For example, in the northern Sierra Nevada where the forest growths are chiefly tall evergreens, that influence must be very different from that exercised by the pine woods in Maine, Colorado, or Georgia, or the stunted growth of acacias in Arizona, In each case the surroundings are influenced by these forests growths—the degree differing according to their constitution and location.
So far as irrigation purposes are concerned, a forest cover may exercise a beneficial influence, by affording a sufficient quantity and an equable flow of water.and by keeping the channels free of debris and silting. But whether or not a forest area may increase the amount of rainfall over its own or an adjoining area is a question that cannot be finally or satisfactorily settled until better means for measuring rain shall have been afforded. At the same time much less water is transferred by tree growth, as a rule, than by the lower annual vegetation or by evaporation caused by sun and wind when the ground is not shaded. The velocity of the winds, it is well known, increases their evaporative powers in accelerated ratio; wherefore, the wind-breaking quality of a forest growth, to say nothing of its powers of shade, causes a large amount of moisture to be retained and this more particularly where the snow is the source of supply—the rapid wasting of
evaporation being thereby noticeably prevented.
Again, the forest floor, owing to the deep reaching root system of the trees keeps the soil in such a granular and open condition as to allow of the water penetrating through it; in open and barren ground the soil is so close grained,—so hardened -that the rain runs off superficially and cannot enter the soil. Here again, however, soil-conditions and the local geological structure of the locality must be taken into consideration. Loose sand is easily permeable; stiff clay, even though underlying the forest, being nearly impermeable. But a permeable soil under such a cover is the parent of subsoil, instead of surface drainage. At all events, the former is afforded a longer time before the latter even begins. Under these conditions, when the snow begins to melt in the spring, the water resulting therefrom can percolate gradually through the forest soil, which is unfrozen and permeable, while the water rushes right off the frozen and hard packed ground, which has been stripped of its forest cover, thereby causing, or increasing the violence of the spring floods.
A permeable soil, added to the obstruction resulting from the trunks of the trees and the shrubbery, not only decreases the amount of the surface drainage, but a!so changes it into subdrainage,besides preventing the washing away of the earth, the formation of gullies, and the carrying of silt and debris into the water channels. In this connection, however, it must not be forgotten that, if the forest floor has been repeatedly burned, if it has been continuously and for a long time exposed to the trampling of cattle or sheep, or if the trees have been nearly all cut down, such conditions as to the forest covering will greatly reduce the permeability of the soil.
This has been shown in France during the last one hundred years—especially during the last thirty years. The deforestation and denudation of some 8,000,000 acres of mountain fftrest land has ruined t,000.000 acres of rich agricultural land in the plain below; the water stages having been suddenly changed, and the torrent-action of the river having covered the ground over with silt to a distance of quite 200 miles away from the original source of the devastation. The hill lands of Mississippi and elsewhere in the United States—the Adironondack district not excepted—bear like witness to the result of allowing such a policy to obtain. Its evil effects have cost France over $24,000,000 in reforesting—three times that amount would not be enough to remedy the mischief; this country having likewise had to pay heavily for its experiences in this line, especially in those districts which depend upon irrigation for the productiveness of the soil.
Hence attention must be paid not only to the preservation of the forests, but also to properly using and rightly conserving the forest cover at the head waters and along the water courses, in such a way as to obtain and maintain the conditions necessary for rendering the water flow a certainty—something that demands a knowledge as well of forestry as of geology. And this course should be followed as far as possible, even where the natural means of securing water from forests are scarce or even altogether absent.
As a first step—and it is a very long stride towards the ultimate attainment of this object, President Cleveland has established thirteen new forest reservation, with an area of 21,000,000 acres—about two-thirds the sizes of New York State —in South Dakota, northern central Wyoming the Teton reserve, adjacent to the Yellowstone Park, Montana, Idaho, Washington, California (northern and southern), and Utah. He has done this on the recommendation of Secretary Francis and a forestry commission of the National Academy of Sciences.